Various photoresist compositions are known in the art of producing printed circuit boards (hereinafter also referred to as PCB's), chip carriers and other circuitized substrate products. When exposed to activating radiation, the photoresist composition is chemically altered in its solubility to certain solvents (developers). Two types of photoresist (also referred to herein simply as “resist”) compositions are known. One is referred to as a negative acting resist which is, generally, a composition which is soluble in the utilized developer but, following exposure to the actinic radiation, becomes less soluble in this developer, thereby defining a latent image. The second type of resist is known as the positive acting resist which works in an opposite fashion to the negative resist in that actinic radiation renders the resist more soluble in the developer. More description of both types is provided herein-below.
The above types of photoresist compositions generally comprise a light sensitive compound in a film-forming polymeric (resin) binder. One resin binder frequently used is an alkali soluble phenolic resin, e.g., cresol-formaldehyde resin. This material is the product of a reaction of cresol and formaldehyde under conditions whereby a thermoplastic (novolak) polymer is formed. Photoresist compositions prepared using the novolak type cresol formaldehyde resin typically incorporate therein a photoactive compound (also referred to as a sensitizer), e.g., one of the group of the 4- or 5-substituted naphthoquinone-1,2-diazidesulfonic acid esters. Examples of various photoresist compositions are provided below.
One photoresist composition used in the manufacture of PCB's is known as “PEPR 2400” photoresist, which is a positive acting waterborne resist (with negative charged micelles) sold by Rohm & Hass Electronic Materials (formerly Shipley Company and Ronel Inc.), having a location at 455 Forest Street, Marlborough, Mass. PEPR 2400 resist may be applied by electrodeposition and provides resolution to as low as five microns. Being a positive acting resist, the developed areas (an alkaline developer is preferably used) of the resist are removed, thereby exposing the defined pattern of the underlying metallurgy (typically a copper foil) on the desired substrate. Rinsing may be accomplished using deionized water and drying performed in a convection oven, e.g., at about 105 degrees Celsius (C). This particular photoresist has been successfully used by the Assignee of this invention to produce PCB's having high density circuit patterns, including those with tightly spaced plated-thru-holes (PTH's) therein. Such circuit patterns and PTH formations are required in many of today's leading PCB products.
More specific examples of various other photoresist compositions are described in the following U.S. Patents. Listing the following patents is not an acknowledgement that any are prior art to the present invention.
In U.S. Pat. No. 7,011,923, there is described a negative photoresist composition and a method of patterning a substrate through use of the composition. The negative composition comprises a radiation sensitive acid generator, a first polymer containing an alkoxymethyl amido group and a hydroxy-containing second polymer. The first and second polymers may also have a repeating unit having an aqueous base soluble moiety. The first and second polymers undergo acid catalyzed cross-linking upon exposure of the acid to radiation, producing a product that is insoluble in an aqueous alkaline developer solution.
In U.S. Pat. No. 6,638,689, there is described a photoresist composition including a photopolymerizable organic material, a water-soluble resin and an amino-group-containing resin, which is applied on the surface of a polyimide precursor layer to form a photoresist layer. Then, the layer is patterned by a photolithographic process. The polyimide precursor layer is etched and the pattern of the photoresist layer is transferred to the polyimide precursor layer. The amino-group-containing resin in the photoresist layer is combined with an acid anhydride in the polyimide precursor layer to attain good adhesion and high-accuracy etching without causing separation of the layer. Alternatively, a film can be formed from the photoresist composition and then applied on the surface of the polyimide precursor layer to form a photoresist layer.
In U.S. Pat. No. 6,110,643, there is described a process for fabricating a multilayer PCB with permanent inner layers of photoresist, the photo-imageable composition used to form the hard permanent layers comprising a polymerizable acrylate monomer, an oligomer formed by the reaction of an epoxy resin and an acrylic or methacrylic acid, a photosensitive, free radical generating initiator for polymerization of the acrylic monomer and oligomer, a curable epoxy resin, a curing agent for the epoxy resin, and, optionally, a crosslinking agent reactive with hydroxyl groups. After exposure and development of a layer of the photoimageable composition and etching of the underlying metal layer on a board, the resultant photoresist is left on the circuitry traces. The inner layers of photoresist are stacked in a press where it initially conforms under heat and pressure to fill voids between the inner layers, and then cures to permanence.
It is to be understood that the photoresist compositions of the present invention may also be retained as permanent inner layers of multilayered circuitized substrates of the type discussed in U.S. Pat. No. 6,110,643, if desired.
In U.S. Pat. No. 5,698,370, there is described a photoimageable composition, useful as a photoresist for forming a printed circuit board, which is alkaline aqueous developable but, subsequent to exposure and development, is processable in highly alkaline environments, such as additive plating baths and ammoniacal etchants. The photoimageable composition comprises between about 25 and about 75 wt % of a binder polymer, between about 20 and about 60 wt % of a photopolymerizeable material which is a multifunctional photopolymerizeable monomer or short chain oligomer, and between about 2 and about 20 wt % of a photoinitiator chemical system, the weight percentages being based on the total weight of these components. The improvement is the use in the photoimageable composition of a binder polymer which is a styrene/maleic anhydride copolymer in which the maleic anhydride residues are mono-esterified to between about 50 and about 65 mole percent of an alkyl, aryl, cycloalkyl, alkaryl, or arylalkyl alcohol having a molecular weight greater than 100, to between about 15 and about 50 mole percent of a C.sub.1-C.sub.3-alkyl alcohol, and to at least about 80 mole percent total. The polymer has between about 45 and about 65 mole percent styrene residues and between about 35 and about 55 mole percent maleic anhydride residues, a weight average molecular weight of between about 80,000 and about 200,000, and an acid number of between about 170 and about 220.
It is to be understood that the photoresist compositions of the present invention may include those of the photoimageable type, such as the one discussed in U.S. Pat. No. 5,698,370.
In U.S. Pat. No. 5,252,427, there is described an aqueous-processable, positive-working resist composition allegedly having improved photospeed and aqueous development rate without substantially reducing processing latitude in printed circuit chemistries. The composition includes a polymeric material having a polymer backbone with pendant acid labile groups which are bound directly or indirectly to the polymer backbone, and free acid groups, wherein the polymeric material has an acid number of about 25 and is substantially insoluble in 1% by weight aqueous sodium carbonate solution at 30 degrees Celsius (C). The composition further includes a substance that forms an acid upon exposure to actinic radiation. These photoresists may be applied as a liquid coating or from an electrodeposition bath.
In U.S. Pat. No. 4,898,656, there is described an electrodeposition coating process of photoresist for printed circuit board manufacture, the process involving electrodepositing on a conductive surface a photosensitive electrodeposition coating composition, and thereafter further electrodepositing on the resulting coating film an electrodeposition coating composition composed mainly of a water-soluble or water-dispersible resin having a glass transition temperature of at least 20.degree. C. The photosensitive electrodeposition coating composition is basically a composition containing as the main ingredients a resin having salt-forming groups for imparting solubility or dispersibility in water, and photosensitive groups.
In U.S. Pat. No. 4,869,995, there is described photosensitive resinous composition which includes a resin dissolved in an organic solvent, examples of the latter including glycol ethers such as ethyleneglycol monoethyl ether, ethyleneglycol monobutyl ether and the like, cellosolve acetates such as ethyleneglycol monomethyl ether acetate, ethyleneglycol monoethyl ether acetate and the like, aromatic hydrocarbons such as toluene, xylene and the like, ketones such as methyl ethyl ketone, cyclohexanone and the like, and esters such as ethyl acetate, butyl acetate and the like. The resinous composition is allegedly useful as a photosensitive material for resist and lithographic use. Since the photosensitive groups are incorporated in the resin molecule, the resin is claimed to exhibit a long-lasting photosensitivity and excellent stability. As the exposing light, various actinic radiations having absorption maximum at different level in about 200 to 400 nm range may be selected and a particularly high contrast image can be obtained with relatively shorter wave length radiations. The authors claim that the composition is useful in the production of resist material for printed circuit boards.
It is to be understood that the teachings of the present invention are applicable to both positive and negative acting photoresist compositions, including the described PEPR 2400 resist. However, the invention is not limited to a particular resist composition, whether positive or negative acting, but may be successfully utilized in many compositions, including those of other types such as discussed above.
It has been learned that certain forms of bacteria may become present in the photoresist composition while in its liquid form, prior to deposition on the desired substrates, and that this bacteria has attacked the photoinitiator(s) present in the composition over a period of time (e.g., during composition storage), thereby limiting the functional bath life of the liquid resist. Because photoresist compositions such as the aforementioned PEPR 2400 resist are considered relatively expensive (especially in comparison to resists with lesser functional capabilities), the problems associated with such bacteria presence can prove to be costly to the PCB manufacturer.
It is believed, therefore, that a new photoresist composition which includes as part thereof means for destroying and/or preventing bacterial growth within the composition will represent a significant advancement in the art. It is especially believed that a composition including such means which is capable of being exposed in such a manner as to enable the formation of high density circuit patterns would constitute a particularly valuable art contribution.